Conventional electrical power systems utilize a synchronous electrical generator for generating AC power. Particularly, such a generator may include a rotor and a stator having a stator coil. In applications such as an aircraft, the rotor is driven by an engine so that electrical power is developed in the stator coil. Owing to the variation in engine speed, the frequency of the power developed in the generator windings is similarly variable This variable frequency power is converted to constant frequency power in a variable speed constant frequency (VSCF) system including a power converter which may develop, for example, 115/200 V.sub.AC power at 400 Hz. Such known converters are controlled by a generator/converter control unit (GCCU).
In order to provide aircraft engine starting, such known power systems have operated the generator as a motor. Specifically, an external power source is coupled through a start control to the generator to energize the stator coil and thus develop motive power to start the engine. The components required in such a start control increase the weight of the aircraft and take up valuable space. To minimize the size and weight of such start controls, certain known aircraft VSCF power systems have utilized the existing converter and GCCU for the start control.
In the start mode of operation, the converter may be supplied power from any 400 Hz power source, such as, for example, an auxiliary power unit generator or an external power source. However, each such power source might have a different available capacity for use in engine starting. Therefore, the GCCU must be configured to provide engine starting from any such available power sources and to limit the amount of power drawn.
Rozman et al. co-pending application entitled VSCF Start System with Selectable Input Power Limiting, Ser. No. 220,625, filed Nov. 14, 1988, and owned by the assignee of the present invention, which is hereby incorporated by reference herein, discloses a start control which provides input power limitations in accordance with input power requirements. Specifically, the start control described therein utilizes a pulse width modulated inverter to control torque and power as functions of the output voltage and commutation angle. Specifically, the start control maintains the volts/hertz ratio at a constant and uses closed loop control of the commutation angle at speeds above a preselected minimum to control current and to limit input power.
In order to control stator current it is necessary to provide feedback information representing actual current. This information is commonly obtained using a current sensing device to measure the actual current. Known measuring systems employ a shunt resistor, a current transformer or a hall sensor. Shunt resistors are practical only for low power motor control due to the additional power losses and weight. Current transformers are typically used to measure larger current levels at medium and high speeds. At low speeds, current transformers are less accurate. To overcome such inaccuracies, the size and weight thereof must be increased. Hall sensors are typically used to measure current at low speed. However, such sensors tend to be sensitive to the temperature range and they are of relatively large size.
Another problem resulting from the use of current sensors is the necessity of filtering the signals produced thereby. At low rotational speeds, a conventional scheme which uses amplitude detection requires a filter with a large time constant which limits the dynamic performance of the motor control system.
The present invention is intended to overcome one or more of the problems as set forth above.